(1) Field of the Invention
The present invention is directed to cables having utility as radio frequency transmission lines and having improved shielding properties.
(2) Description of the Prior Art
It is known that in many applications a conventional cable having a center conductor surrounded by a single flexible coaxial sheath does not have sufficient shielding properties to provide adequate suppression of EMI or RFI interference. Accordingly, in another conventional cable a second flexible coaxial sheath which is a good conductor is positioned in concentric relation to the first coaxial sheath which is also a good conductor. These two sheaths are either in electrical contact or separated by an interlayer of dielectric material having a relatively low dielectric constant and a low dissipation factor. When this interlayer dielectric is used, the construction is commonly called a triaxial cable. In this conventional triaxial cable, the coaxial sheaths are separated to increase the series impedance of the path between the sheaths thereby improving radio frequency shielding. However, use of a dielectric material having a relatively low dielectric constant and a low dissipation factor results in a small propagation function (propagation constant) in the path between the sheaths thereby resulting in the shielding performance being length dependent. In such a cable the ratio of the propagation function in the path between the two sheaths and the propagation function in the path between the center conductor and the inner sheath is less than about 2.
Conventional cables utilizing more than two sheaths in electrical contact or with an interlayer of dielectric material having a relatively low dielectric constant and low dissipation factor or combinations of the same, are used to further improve the shielding. Some cables additionally employ metallic armors for mechanical protection of the cable and/or drain wires for ground connection which are laid over or under the coaxial sheath or sheaths.
In a conventional cable, the sheath or sheaths are made from conductive material such as, for example, braided conductive wire, solid metallic sheath, solid metallic tape, or laminate tape formed of metallic and plastic layers. Braided sheaths, typically made from braided aluminum or copper wire and having an optical coverage of greater than ninety percent of the surface area of the sheath, are used as shields to obtain more mechanical flexibility than is achieved with a solid sheath. However, the shielding of the braided sheaths is inferior to that of a solid sheath and results in a higher propagation attenuation of the internal Transverse Electromagnetic (TEM) signal due to an increase in the power loss (I.sup.2 R loss) of the sheath. To improve the shielding of a cable, a plurality of braided sheaths are typically used.
The relatively low propagation attenuation achieved by using a solid conductive sheath can be obtained by using a laminate metallic and plastic tape as the inner sheath. A cable made with a laminate metallic and plastic tape has increased flexibility in comparison to a cable made with a solid metallic tape sheath. The laminate tapes have one or more very thin metallic layers adhered to thin plastic layers. The laminate tapes may be bonded or adhered to the adjacent parts of the cable. Compared to braided sheaths the laminate tape generally offers inferior low frequency shielding and superior high frequency shielding. More than one layer of laminate tape may be used to improve the shielding and drain wires may be laid over or under the laminate tapes to provide termination to the connector.
A combination of braided shields, solid metallic tapes and laminate tapes are used to improve the shielding. In many conventional cables more than two sheaths are required to provide sufficient shielding, resulting in an appreciable increase in cost and decrease in flexibility of the cable.